The Collegia Titanica is the section of the Adeptus Mechanicus that includes the Imperial Titans, the colossal god-machines. The Collegia is also known as the "Adeptus Titanicus" (a contraction of "Adeptus Mechanicus Collegia Titanica")
During the Horus Heresy, part of the Collegia turned against the Imperium along with almost half of the Adeptus Mechanicus. Out of all the Collegia's Titan Legions, only those on Terra remained loyal. The rest declared for Horus. The rebel Titan Legions even took part in the siege of the Imperial Palace. At the defeat of the rebellion, the rebel Titans were driven into the Eye of Terror, along with the rest of those who had sided with Horus.
The Collegia has four divisions: Divisio Mandati, Divisio Telepathica, Divisio Investigatus, and most importantly, the Divisio Militaris which actually includes the Battle Titans. The Divisios are further divided into a number of "Legions" or "Orders": groupings of Titans such as the Fire Wasps Legion or War Griffons Legion. Each Legion is a self-contained unit headed by a Grandmaster.
The Military Orders
The Orders or Legions of the Divisio Militaris (or more simply the Military Orders) are the main military forces of the Collegia, and each, as self-sufficient units, includes its own ground combat troops and support staff in addition to its Titans.
Each Legion is based on its own Forge World - most Forge Worlds having one (or in some cases several) Titan Legions.
Legion detachments are stationed throughout the galaxy, guarding vulnerable locations across the Imperium. Many Legions are stationed near the Eye of Terror, ready to combat their Chaotic counterparts.
Legion strength is typically around 50 operational engines.1 Battle losses can take centuries to replace, as the construction of a new Titan is a long and laborious process. The Warlord is the most commonly found class while the Imperator is by far the most formidable but an increasingly rare sight on battlefields. Though there have been rumours of a stronger class that existed during the Great Crusade (possibly the Apocalypse class).
Titan Legions are deployed only in the most vital campaigns and on battlefields that will allow them the room to manoeuvre and unleash the full power of their colossal weaponry.
The Titan Orders of the Mandati are known as "Executive Orders". They travel in vast temple-spacecraft, responsible for bringing the Pax Imperia to independent human worlds. Each temple ship carries a small but potent number of the immense Emperor Titans, plus priests, regular troops, maintenance adepts and support personnel.
Each Emperor Titan is its own self-contained unit, carrying members of the Adeptus Arbites, the Inquisition and the Adeptus Terra. It is through this combination of persuasion and vast firepower that many worlds are brought into the fold of the Imperium.
The elite of the Mandati's orders are known as "Missionary Orders" - they travel beyond the known areas of space, often accompanying Rogue Trader missions.
As a highly specialised Divisio, the Telepathica is somewhat smaller than the Mandati and Militaris Divisios. The Telepathica is responsible for the dreaded Psi-Titans. Its orders are the Telepath Orders, each said to operate from a secret forge world near the heart of the Imperium.
The Investigatus is the research division; its role is to construct the Titans of all the orders, and to engineer and test improvements in technology. All of the Investigatus orders are based on Mars, the capital Forge World. At times a research order takes part in battles in order to field test new designs.